1. Field of the Invention
The present invention relates to a lithium rechargeable battery and a lithium rechargeable battery pack that has improved over-discharge characteristics.
2. Discussion of the Background
Rechargeable batteries with a high power storage capacity are being developed to power small, high power electronic devices such as video cameras and portable stereos. Lithium rechargeable batteries may use carbonaceous materials as a negative active material and a lithium cobalt oxide (LiCoO2) as a positive active material. The lithium rechargeable batteries typically exhibit high energy density, high power storage capacity, and good cycle life characteristics.
Many lithium rechargeable batteries undergo deterioration of charge characteristics at over-discharge. When an electronic device malfunctions or uses a battery that has a cut-off voltage that is not set for the electronic device, open circuit voltage (OCV) is not restored even though a circuit is open when a discharge voltage is 0 V. Then when the battery is charged and discharged, the battery capacity may deteriorate significantly and may result in an internal short-circuit. In this way, charge and discharge characteristics in case of over-discharge to 0 V are very important in the commercialization of a rechargeable battery. Therefore, the deterioration of charge and discharge characteristics may be alleviated.
Performance deterioration and shortening of cycle-life upon over-discharge result from dissolution of copper from the negative current collector. Dissolution is induced when the potential of copper increases to 3.5 V (vs. Li) or when the operation potential of a positive electrode at a final step of over-discharge exceeds the deposition dissolution potential of 3.45 V (vs. Li).
Lithium rechargeable batteries are often equipped with an over-charge protection circuit that can stop discharge of lithium rechargeable batteries when an electronic device malfunctions and prevent loss of battery capacity due to over-discharge. Generally, such a protection circuit may protect from both over-charge and over-discharge (e.g., Japanese Patent Laid-Open No. Hei 9-63652). However, these conventional lithium rechargeable batteries require a means to install the over-discharge protection circuit. In addition, an over-discharge protection circuit must be individually designed for each electronic device.